Researchers pave way for much brighter OLEDs

Controlling electron spin is key to improving OLED panels.

The two poles of an electromagnet control the orange glow from a spin-organic LED.

Tho Nguyen, University of Utah

Researchers have built the first organic LED (OLED) that is controlled by the spin of the charge carriers running through the device, paving the way for future OLED devices to offer increased brightness. Though rumors say Samsung will release a 55-inch OLED TV this fall, don’t expect a spin-OLED on the market soon. This prototype orange OLED only works at temperatures below -33°Celsius (-28°Fahrenheit).

OLEDs contain layers of organic polymers sandwiched between two electrodes. (Organic here refers to molecules containing mostly carbon, hydrogen, oxygen, and nitrogen, like those in our bodies.) These polymers act like semiconductors, so applying voltage across the sandwich generates electrons at one electrode, and their positive partners, called “holes,” at the other. These electrons and holes travel along the polymers, smashing together when they meet. This collision pumps energy into the molecule. It loses that extra energy by emitting light.

But there’s a catch. Polymers only emit light when the spins of the electrons and holes are arranged in particular combinations. Think of spin as a tiny bar magnet inside the electrons and holes. When two spins meet, the north poles of each spin can point the same direction, or they can oppose each other. Both of these combinations can create light, though whether or not they both do it in the same organic semiconductor depends on the individual polymers.

In an OLED that emits light when the electron and hole spins are antiparallel, only one in three combinations creates light. Z. Valy Vardeny (at the University of Utah) and his colleagues wanted to boost the efficiency of an OLED by controlling the spin of electrons and holes as they’re pumped into the polymer. They increased the number of carriers with the proper orientation to emit light as they combine. This naturally boosts the brightness of the OLED, Vardeny says.

The scientists do this using a device called a spin valve, which was first developed to control current in 2004. This spin valve sandwiches an orange light-emitting organic polymer between two ferromagnetic electrodes. One electrode, a strip of cobalt covered with lithium fluoride, injects electrons into the polymer. The other, lanthanum strontium manganese oxide (LSMO), reliably injects spin-aligned holes. The scientists control the spin alignments of the two electrodes—and thus the spins of the charge carriers coming from those electrodes—using an external magnetic field.

To slow spins from flipping as the carriers moved along the organic semiconductor, the researchers replaced all the hydrogen atoms in the polymer with deuterium (an isotope of hydrogen that has one extra neutron). And when they applied about 3.5 V and an external magnetic field to the 300-micrometer square device, the OLED glowed orange.

Until now, spin valves have only been used control current flow because they could only inject holes, and not electrons, into a polymer. Coating the cobalt electrode with lithium fluoride turns that electrode into an electron source. Since the other electrode pumps out holes, the scientists can create the two carriers that combine to generate light.

With the right polymer, it might be possible to control the color of a spin-OLED using a magnetic field, Vardeny says. Currently, the different colors of an OLED are due to different polymers. Or perhaps one day an entire spin-OLED—including the ferromagnets—could be made of organic molecules, he adds.

How does a hole, which is basically a place for an electron to go, have a spin? Is it that all the other electron spin states in the molecule are taken, leaving only one possibility for an incoming electron?

Polymer requires the use of deuterium instead of hydrogen. This thing is not going to be cheap. The temperature requirements are a bit of a hassle as well but that may be able to be engineered out, but if it requires deuterium that may be a show stopper. Or maybe not, I have no thought of how much deuterium costs, but I do remember having to get special isotope-based materials for experiments way back when was very expensive.

I suppose the actual brightness will be the same, and they'll trade it off to use less energy.

Thanks,Karl

I'm wondering this, too. Is this merely allowing for increased maximum brightness or have they developed a method to significantly increase efficiency? Not clear to me based on the content of the article.

Well I doubt we'll ever see this used in an actual consumer OLED display since you can harvest triplet and singlet excited states with metal dopants...

But it is interesting that the spin state of the electrons/holes/exciton (usually approached from a quantum mechanics viewpoint) is being controlled by a magnetic field (a macroscopic approach). I guess if scientists produce cheap organic magnets this could be useful...

I confess a certain prejudice against this technology, purely because of the word "organic" which to me implies a degree of degradation over time. And, as it happens, OLEDs do actually degrade over time. I wonder if it'll be possible to replace all the carbon in these molecules with silicon eventually.

Or they could use an iridium-based emitter to harvest both triplet and singlet excitons.

After what, hauling both Vesta and Ceres into orbit to provide the kind of iridium tonnage we'd need to supply the OLED market?

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OLED tech is the only viable long term solution for optical devices. Iridium is far too precious.

Working in this field myself, I can tell you that "organic" is not a very strict definition. It includes silicon, boron, as well as the incorporation of the very common metals (like iron, copper, etc). The differentiation is targeted specifically at the expensive metals which in include Ir, Pl, Pt, Rh, Ru, Os, Re, etc.

The long, long term goal is to further increase viability by swapping to bio-plastics which are based on crop sugars as a main carbon source instead of petroleum based varieties that are heavily relied on in most of our goods, both electronic or otherwise. But the tech there still has a long way to go, it is catching up though =)

I suppose the actual brightness will be the same, and they'll trade it off to use less energy.

Thanks,Karl

I'm wondering this, too. Is this merely allowing for increased maximum brightness or have they developed a method to significantly increase efficiency? Not clear to me based on the content of the article.

It boosts efficiency and brightness. For an OLED that works on fluorescence (spins antiparallel), I'm told the spin-OLED is twice as efficient.